No support for anything "inevitably" happening. We have statements that tell us what will happen if certain conditions occur, such as if plants are growing quickly, or very slowly, or if there's a substantial increase in CO2 released. But we have no evidence that any of these conditions actually occur or will occur.

This is a very tempting trap answer. You might be thinking that if there's a substantial increase in CO2 released, plants in the tropics would grow more slowly, which seems to trigger the second conditional, allowing us to conclude that the bulk of the plants on Earth (which are in the tropics) will take in less CO2 than they release. However, "grow more slowly" is a relative claim that doesn't clearly put the rate of growth into the absolute category of "growing very slowly." The plants in the tropics, even if they grow more slowly as a result of temperature increase following a substantial increase in CO2 in the atmosphere, could still be growing quickly (or at some other rate that is faster than "very slowly"). They just wouldn't be growing as quickly as they used to.

Supported by the causal chain in the last two sentences. We know that a substantial increase in CO2 released into the atmosphere leads to a temperature increase in the tropics, which in turn leads to plants in the tropics to grow more slowly on average. This supports (and arguably even logically proves) the claim that if plants release enough CO2 to cause a substantial increase in CO2 released into the atmosphere, plants in the tropics will grow more slowly on average.

(Note that this answer isn't asserting that plants will release enough CO2 to substantially increase the amount in the atmosphere, which is a claim that wouldn't be supported. The answer makes a claim about what would happen if plants release enough CO2 to substantially increase the amount in the atmosphere. And that's something we do have evidence of, because of the third and fourth sentences.)

If you think (D) is supported by the last sentence, you're interpreting that sentence backwards. We know that a temperature increase in the tropics leads to slower plant growth there. But this doesn't imply that a temperature increase will follow any decrease in tropical plant growth speed. Plants might grow more slowly for other reasons besides a temperature increase, even if a temperature increase is one condition that would lead to slower growth.

If you think (E) is supported by the third sentence, you're interpreting that sentence backwards. We know that a substantial increase in the amount of CO2 released leads to a temperature increase in the tropics. But this doesn't imply that a temperature increase must be caused by an increase in CO2 in the atmosphere. Temperature in the tropics might increase for other reasons besides CO2, even if an increase in CO2 is one thing that can cause higher temperatures.